More specifically, the purpose of this invention is to improve the technology of pulverizing coal for burning in electric power generation boilers. This is done with a machine that is basically a system of spinning counter rotating rotors uniquely combined with means for electrostatically and/or aerodynamically separating the fine pure coal from the pyritic and other impurities.
As chunks of coal are fed in through an axial center mounted feed tube, they are caused to smash repeatedly, at high velocity, onto other coal chunks and particles which have accumulated on the rings. By having the coal particles themselves act as the primary abrasion and reduction agents, material wear is minimized. Reduced in size from the series of abrasive collisions, the particles finally exit as an evenly dispersed circumferential spray of very fine material. At this point in the process, an in-stream aerodynamic and/or electrostatic separation action can readily be utilized to remove a high percentage of the sulfur and iron pyritic impurities contained therein.
Currently used pulverizing technology uses direct crushing means such as hammer mills, ball mills or roll mills of various configurations. In these mills, air is swept through the mill and as the coal is reduced to a fine enough size to be airborne the dust particles are entrained in the air stream and carried out of the mill to the combustor.
For material to leave the mill it has to stay in the mill until it is reduced to dust fine enough to become airborne by repeated crushing actions of the rolling or flailing elements of the mill. Pure coal and impure coal both leave the mill when ground down fine enough to be swept up by the air currents blowing through the mill. Therefore, significant separation of pure and impure coal does not take place in these types of reduction mills.
When coal is mined, it often carries impurities mixed in its seams in the form of streaks ranging from small fractions of an inch to several inches in thickness. These stratified streaks of impurities are chiefly composed of both iron pyrites and sulfur, and when intermixed with the coal, comprise what is known as "bone" coal. Sulfur can also appear as chunks called "sulfur balls". The large ones are taken out at the mine, but some small ones may get through. The bone coal is approximately three and a third times more dense and considerably harder than pure coal. Being harder, the bone coal requires greater energy in the form of collisions to reduce to dust in conventional mills. Yet, the mechanical crushing elements found in these types of mills do eventually reduce the bone coal to a fine enough size to be carried out to the boiler burners by the air sweeping elements.
Thus, this conventional system of reduction offers a major drawback since the reduction of bone coal in these mills is not only useless, but the additional crushing power required to reduce the bone coal as well as the metal on metal contact produced therein results in high amounts of wear on mechanical parts. The present invention seeks, as one of its purposes, to use a means of reduction that will break down the soft friable coal but not crush the hard bone coal as much. This reduction process will reduce the pure coal to dust form and leave the impure coal in relatively larger, harder, and heavier chunks so that a simple separation process that recognizes these different characteristics will reject the bone coal, with its impurities, before it can be carried to the combustors.
The construction and operation apparatus and system will be described for pulverizing the coal. Also, two means will be shown for separating out the impurities, followed by size classifying means that will separate combustible size coal dust and oversize chunks that are returned to the mill for further reduction.
The use of this unique system of fuel preparation makes it possible to utilize in power generation and heating plants the so called high sulfur coals from the eastern states without high pollution effects on the atmosphere.